Fuel control system for a gas turbine

ABSTRACT

A system for controlling the quantity of fuel supplied to the combustion chambers of a gas turbine during the ignition and operating periods, wherein a fuel signal is developed from at least one turbine operating parameter, to regulate a fuel control valve. The signal flows through a first alternate path during the ignition period and a second alternate path during the operating period. A diverting device switches the signal from the first path to the second path upon response to a predetermined level of the control signal. A limiter device is provided in the second path to maintain a minimum signal to the valve corresponding to the level required to maintain combustion.

rated States Patent 1 11 3,729,929 Kiscaden 1 May I, 1973 1 FUEL CONTROLSYSTEM FOR A GAS 3,630,023 12 1971 Lazar ..60/39.28 TURBINE 3,310,9373/1967 Smith ..60/39.28 [75] Inventor: Roy W. Kiscaden, Springfield, Pa.Primary Examiner camon R Cmyle [73] Assignee: Westinghouse ElectricCorporation, Assistant Emminr-R0befl a r t Pi b h p Att0rney-A. T.Stratton, F. P. Lyle and F. Cristiano [22] Filed: Mar. 9, 1971 57ABSTRACT [21] Appl- 122,427 A system for controlling the quantity offuel supplied 1 to the combustion chambers of a gas turbine during [52]U.S. Cl. ..60/39.28 R the ignition and Operating periods wherein a fuel51 Int. Cl. F021: 9/06 Signal is devemped mm least one turbine ("mating[58] Field of Search 39 28 R parameter, to regulate a fuel controlvalve. The signal 6'0/3928 flows through a first alternate path duringthe ignition period and a second alternate path during the operatingperiod. A diverting device switches the signal from [56] ReferencesCited the first path to the second path upon response to a UNITED STATESPATENTS predetermined level of the control signal. A limiter device isprovided in the second path to maintain a 2,986,880 6/1961 Fortmann..60/39.14 minimum signal to the valve corresponding to the Bayard 14level required to maintain combustio 3,151,452 10/1964 Bunger et a1...60/39.14 3,151,450 10/1964 Blackaby ..60/39.28 13 Claims, 5 Drawing Fiures 21 FUEL 22 LTRANSDUCERJ/ wiQ, F 11 "aiiiL/ LIMITER DIVERTOR I ,46 F1 44 i L:- l il Patented May 1, 1973 R 3,729,929

2 Sheets-Sheet 2 25 T s? 0 J TEMP I HQ 2 290 SURGE i 32 I320 I l 7 POWERr l I I SPEED ERRoR REFERENCE DETECTOR I as l FIG. 5

WITNESSES INVENTOR JW/A/W/ Roy w. KiscgiZ BACKGROUND OF THE INVENTIONBecause of the increase in peak power requirements, industrial gasturbines of the axial flow type have increased substantially in size andpower output. This requires complex controls to regulate larger andheavier equipment. For example, a control system is provided to regulatethe quantity of fuel supplied to the combustion chambers of the gasturbine. Large and.

heavy valves (for example weighing approximately 200 pounds) are used torestrict the flow of fuel to the turbine. These valves are difficult toaccurately control and set, especially during the initial installationperiod of the valves. Furthermore, the valves add to the larger size ofthe turbine power plant, and there is a corresponding increase in costassociated therewith.

Generally, there is a large starting valve which is responsive to acontrol signal developed from turbine operating parameters, such asrotor speed, combustion chamber pressure, power output, and turbineexhaust temperature. After the initial cranking period of the gasturbine, the starting valve is used to control the amount of fueladmitted into the combustion chambers during the ignition period, untilthe quantity of fuel supplied reaches the minimum quantity necessary tosustain combustion. The starting valve is generally designed to give alow gain output necessary for accuracy during the starting cycle of theturbine, i.e. a large signal change would produce a smaller change inthe lift or opening of thevalve. This valve also functions to maintain aminimum amount of fuel supplied to the combustion chambers to supportflame during a transient condition, such as from loss of load.

A second valve, such as a throttle valve, is responsive to a secondcontrol signal generated from the parameters stated above. The throttlevalve controls the quantity of fuel supplied to the combustion chambersafter the minimum quantity to sustain combustion is reached andcontinues to supply the quantity of fuel needed for the turbine to runat its full operating condition. Characteristically, the large throttlevalve is inaccurate at the low flow rate levels.

It would be desirable to design a fuel control system which wouldeliminate the necessity of a starting valve used in the starting systemand a throttle valve used in the normal operating system and to replaceit with a less expensive control system which would help reduce the sizeof the turbine plant. It would further be more desirable to simplify thecontrol settings on the new control system.

SUMMARY OF THE INVENTION A system for supplying fuel to the combustionchambers of a gas turbine, providing a fuel valve, which is responsiveto control signals generated during the ignition period and during theoperating or running period. The signals are generated by turbineparameters, such as rotor speed, combustion chamber pressure, poweroutput, and/or turbine outlet temperature.

After the initial low speed cranking period, a control signal or fuelscheduling signal is generated and sent through a first alternate orstarting signal path to a twoposition diverting device. The signalcontinues through the first position of the diverting device and goesthrough a common signal path to the valve, to activate the valve andregulate the flow of fuel.

A sensing device is connected to the common signal path. When thecontrol signal reaches a value corresponding to the minimum level offuel supplied to the combustion chambers to sustain combustion, thesensing device activates the diverting device to the second position.The signal is then carried to the valve through a second alternate ornormal operating path, the second position of the diverting device andthe common path.

A limiter device is secured to the second signal path. Once the signalgoes through the normal operating path, the limiter output signal equalsthe control or fuel scheduling signal unless, during a transientcondition, the limiter input signal or fuel scheduling signal goes belowthat required to sustain combustion. During such a transient condition,the limiter maintains a signal to the fuel valve at a level required tosustain flame in the combustion system. The limiter maintains thissignal level until the fuel scheduling signal becomes equal to orgreater than that required to sustain flame. When the fuel controlsignal becomes equal to or greater than that required to sustain flame,the output of the limiter equals the fuel scheduling signal, thusinsuring that combustion is maintained even if radical changes in theparameter occur.

After turbine shut down, the limiter device is bypassed by switching tothe starting path by the diverting device to allow a proportionalcontrol signal, which is less than that required to sustain flame, toreach the valve corresponding to the ignition period.

This fuel system thus eliminates the starting valve, reduces the overallsize of the turbine plant and the costs associated therewith, andsimplifies the control settings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammaticrepresentation of a fuel control system for a gas turbine incorporatingthe invention therein;

FIG. 2 is a diagrammatic representation of the signal comparing meansshown in FIG. 1;

FIG. 3 is a diagrammatic representation of the limiter device shown inFIG. 1;

FIG. 4 is a diagrammatic representation of a signal diverting deviceshown in FIG. I; and

FIG. 5 is a diagrammatic showing of the sensing device shown in FIG. 1.

DETAILED DESCRIPTION OF Til-IE PREFERRED EMBODIMENT Referring to thedrawings in detail and specifically to FIG. 1 there is shown adiagrammatic representation of a single shaft axial flow gas turbine 10.The turbine 10 includes a compressor 11, a combustion chamber orcombustor l2, and a turbine 13, the turbine rotor and compressor rotorsbeing connected by a common shaft 15, the turbine driving the compressoras well known in the art. The shaft 15 extends from the compressor 11and is connected to a load 16, which may be a generator. A fuelreservoir 18 supplies fuel through conduit 19 to the combustion chamber12 where it is mixed with pressurized air from the compressor 11 to forma combustible mixture, ignited and burned to produce hot gases whichexpand to drive the gas turbine 10.

The rate of flow of the fuel through conduit 19 to the combustionchamber I2 is regulate-d by a combined starting and throttle valve 21.

A plurality of signals are generated from various turbine parameters toproperly determine the fuel control or fuel scheduling signal. One suchsignal is obtained from a thermocouple device 23 which is used tomeasure the average temperature of the spent gases at the turbineexhaust 24a and in the blade path 24b. The signal corresponding to thetemperature of the turbine exhaust 24a and/or blade path 24b is thensent to a comparator device generally designated 26 and is one inputthereto. The temperature signal is an electrical signal transmittedthrough line although other types of signals may be used and othertemperatures measured at various points.

A second input signal is extracted from the pressurized air within thecombustion chamber 12. The value of the pressure is sensed by a pressuresensing device 28 which generates an electrical signal which is sent vialine 29 to the comparator device 26 as a second input thereto. A thirdinput is a temperature signal sensed by a thermocouple device 30 whichmeasures the compressor inlet temperature and provides an electricalsignal which is sent via line 30a to the comparator. A fourth input is apower signal detected by a power measuring device 31, which is connectedto the load or generator 16 and generates an electrical signalproportional to the power and sent through line 32 to the comparatordevice 26. A fifth input signal to the comparator device is a speedsignal which is measured by a speed signal generator 34. The generator34 emits an electrical signal proportional to the rotational speed ofthe shaft 15 and directs it through line 35.

The five signals are fed into the comparator device 26 where theinformation in the signals is processed and an output signal isgenerated. The output signal is a continuous function of the amount offuel required at that instant. The signal output as shown is electrical,although it is understood that either pneumatic or hydraulic signals canbe used as an input and/or output thereof.

The electrical signal can then be transformed to a pneumatic orhydraulic signal by a transducer 41, if desired.

The control signal during the ignition period, flows through a firstalternate or startup signal path 44 to a two-position signal divertingdevice 48. The signal flows through the diverting device 48 into acommon path 47 and continues therethrough to the combined starting andcontrol valve 21. The control valve 21 may have a pneumatic positionerdevice 22 to properly and accurately position the lift of the liftingrod or valve plug 21a of valve 21 in the sensitive, initial controlpositions. The valve 21 then allows the appropriate amount of fuel fromreservoir 18 to flow through conduit 19 to the combustion chamber 12. Itis noted that valve 21 may be any type of fuel valve which changes inresponse to the control signal.

Upon reaching a predetermined pressure value, a signal sensing device 57is activated and sends an elec trical signal to the diverting device 48through line 51. The diverting device 48 is activated and blocks theflow of the signal in the first alternate or startup path 44 and allowsthe signal through a second alternate or normal operating path 46. Alimiter device 49 is secured to the second alternate path 46. When forexample, there is a larger change in the measured turbine parameters,the fuel control system tends to adjust for that change and a controlsignal responding to the large change in parameters is generated. If thesignal to correct is below the minimum signal to maintain combustion,(may tend to over compensate) then the limiter 49 generates a minimumpredetermined control signal.

FIG. 2 diagrammatically illustrates a preferred comparator device 26.The device 26 comprises a digital computer 37 into which the temperaturesignal from the turbine 13 (in line 25), the pressure signal from thecombustor 12 (in line 29), the temperature signal from the compressor 11(in line 30a) and power signal from the generator 16 (in line 32) arefed. The programmed computer 37 performs a temperature limit function250, a surge function 29a (from the signals of the compressor inlettemperature and combustor pressure) and a power limit or controlfunction 32a. Then a low select function 30 is performed by theprogrammed computer 37 to select the fuel limit signal from thetemperature, surge, and power functions 25a, 29a, and 32a, respectively.The computer 37 emits a speed reference func tion 33 which is fed intoan error detector 33a, external of the computer. The speed signal (inline 33) is also fed into the error detector 33a to determine an errorspeed signal, which in turn is fed into a low limit circuit 36 where itis compared with the fuel limit signal from the computer 37. The lowvalue is selected and the fuel demand signal exits from the comparatordevice 26. A more detailed explanation of the comparator device 26 andparameter measurement can be found in the following copending cases:

Giras and Reuther, Ser. No. 82,470 filed Oct. 20,

1970 (Westinghouse Case No. 40,062) Yannone and Kiscaden, Ser. No.82,469 filed Oct.

20, 1970 (Westinghouse Case No. 42,086) Reed and Rankin, Ser. No. 82,467filed Oct. 20,

1970 (Westinghouse Case No. 42,477) all assigned to the same assignee asthis invention. Other types of comparator devices can be used, such aspneumatic and hydraulic, and the device may also be an analog devicerather than a digital, or may be in any combination thereof.

Referring to FIG. 3, it can be seen that the preferred limiter device 49comprises an independent pressurized pneumatic source 50 connected to alow limit relay device 51 via conduit 52. The relay device 51 comprisesa spring adjusted pressure regulator 53, for setting the minimum signallevel required to sustain combustion and two check valves 55 and 56.Check valve 55 is between a jointure 54 and the pressure regulator 53and the second check valve 56 is inserted in the normal operating path46 before the jointure 54.

A preferred signal diverting device 48, as shown in FIG. 4, comprises acoil structure 58 creating a magnetic field within coil 59, ahorizontally extending metallic rod 61 joined to one end ofa verticallyextending rod 62 at pivot 61a. The other end of rod 62 is secured to thebase portion of housing 64 of the diverting device 48 and is pivotedthereto at pin 65. A shaft 66 extends horizontally from the bottomportion of the rod 62 and a plug 67 is secured to the end thereto, theplug being outside of the housing 64. On the middle portion of the rod62, a second shaft 68 is secured thereto with a plug 69 on the endthereof within the housing 64. The housing 64 has an input port 71 forthe startup signal path or first alternate path 44, an input port 72 forthe operating path or second alternate path 46, and an output port 74.In one position, when the coil 59 is energized, the horizontal rod 61and vertical rod 62 move from left to right, the plug 67 closes theinput port 71 and plug 69 opens the input port 72. In the otherposition, (which is that shown in FIG. 4) the coil 59 is deenergized anda spring 70 returns the rod 61 from right to left, plug 69 closes port72 and plug 67 opens port 71. The signal exits through output port 74into common path 47. The divertor 48 as shown is highly diagrammatic andthe ports71, 72, 74 are sealed to prevent leakage of air.

As shown in FIG. 1, the signal sensing device 57 is connected to thecommon path 47 by path 76 and is connected to the diverting device 48 byline 51. Referring to FIG. 5, it can be seen that the sensing device 57is preferably comprised of a housing structure 77, a diaphragm structure78 responsive to pressure in com mon path 47, and a position sensitiveswitch 36 having an open-close contact 81 (FIG. 4). When the diaphragmstructure 78 engages the switch 80, the contact 81 closes allowing anelectrical signal to be sent to the signal diverting device 48, therebyenergizing the coil 59. Switch 80 may be any type of pressure responsivedevice.

The detailed operation of the control system is as follows. During thecranking period the rotor is brought up to ignition speed. Then duringthe ignition period, the turbine parameters previously described, arefed into the comparator 26 and an output is generated as a continuingfunction ofthe amount offuel required. The fuel control signal in thepreferred system is electrical so it is converted to a pneumatic signalin the transducer 41, and the control signal enters path 43.

During the ignition period (FIGS. 45), the signal in common path 47 isnot of sufficient magnitude to activate the switch 80 and therefore, thediverting device 48 is not energized, so inlet port 72 is blocked andport 71 is open. The signal coming from the transducer 41 in path 43(FIG. 1) enters the first alternate or startup signal path 44 andcontinues through inlet port 71 in the diverting device 48. The signalexits through the outlet port 74 into the common path 47. The controlsignal continues through common path 47 to activate the valve 21(FIG. 1) to allow the proper amount of fuel to flow from the reservoir18 in conduit 19 to the nozzle in the combustion chamber 12.

The fuel control signal flowing through path 47 increases in valueduring the ignition period to a predetermined level corresponding to theminimum level necessary to sustain combustion. When sufficient pressurefrom the pneumatic control signal is sensed by sensor 57, switch 80 isclosed and coil 59 is energized. The divertor 48 switches to the secondalternate or operating path 46 by causing plug 67 to close the inputport 71 and correspondingly cause plug 69 to open port 72. The controlsignal in the first alternate signal path 44 is blocked and the signalflows through the second alternate or normal operating path 46.

As best seen in FIG. 3, the signal flows through signal 4 path 46 in thelimiter 49 and past the one way check valve 56 up to the jointure 54 andout through signal path 46. Then as best seen in FIG. 4, it goes throughport 72 in diverting device 48 and into the common path 47 to the valve21 to regulate the flow of fuel therethrough. However, a pneumaticsignal from pressure source Sill (FIG. 3) enters the regulator 53, whichis set to that signal level to provide a fuel flow sufficient to sustaincombustion, through path 52. The signal exits from the regulator 53 andencounters the check valve 55. When the pressure from the pneumaticsignal in flow path 46 is greater than the pressure in path 52, thecheck valve 55 remains closed and valve 56 is open. However, duringconditions resulting in rapid changes in the value of one or more of thepreviously described operating parameters, the control signal may dropbelow that which is necessary to sustain a flame in the combustionchamber 12. This value is predetermined and the regulator 53 iscorrespondingly set, so that if the pressure in path 46 drops below thatpredefined value, the pressure in path 52 will be greater than thepressure in path 46. Therefore, the signal from path 52 opens checkvalve 55 and the signal therethrough flows into path 46, therebysustaining the minimum level to prevent flameout. The control signalfrom path 52 is prevented from flowing back through line 46 by checkvalve 56, because of the pressure differential, and conversely, checkvalve 55 prevents the control signal in path 46 from flowing into thelow limit relay 51.

The condition of minimal signal flow continues until the signal levelreturns to normal value, so that the pressure on the input of valve 56is greater than the minimum pressure control signal on the output sidethereof. When this condition occurs, check valve 55 is forced to close,the minimum pressure signal is cut off, and the signal from the normaloperating parameters in signal path 46 is the control signal regulatingthe fuel valve.

It is noted that although a pneumatic signal is used from the transducer41 to the combined valve 21, electric and hydraulic analogies can bemade with equipment sewing the same function i.e. a low limit device, adiverting device, and a level sensing device.

Once the signal is switched to the normal operating path 46 it cannot bereturned to the startup path 44 under operating conditions. Of course,during turbine overhaul or shut down, the control system reverts back toits initial position.

What is disclosed then is a fuel control system which processes theturbine control parameters and generates an output fuel scheduling orcontrol signal which is a continuous function of the amount of requiredfuel. During the ignition period, the control signal is sent through thestartup path 44 and the diverting device 48 to regulate the fuel valve21. Upon reaching a preset level, the sensing device 57 activates thediverting device 48 to its second position, to allow the control signalto flow through the normal operating path 46. The control signalcontinues therethrough and upon falling below a predetermined level, aminimum signal is generated by the limiter device to maintain a minimumsupply of fuel to the combustor 12 to prevent a llameout.

Although only one embodiment has been shown it will be obvious to thoseskilled in the art that is not so limited, but is susceptible of variouschanges and modifications without departing from the spirit thereof.

What is claimed is:

I. In a gas turbine comprising at least one combustion chamber and arotor structure, said rotor structure being rotatable at a low speedduring a cranking period, at an intermediate speed during an ignitionperiod, and at a high speed during an operating period, and a system forcontrolling fuel supplied to said combustion chamber, said controlsystem comprising:

valve means for regulating the flow of fuel to said combustion chamberduring said ignition and operating periods;

means responsive to at least one parameter of turbine operation forcontinuously determining the fuel flow required by said combustionchamber and for providing a signal representative of said fuel flow;

means for applying said signal to said valve means to effectuateoperation of said valve means to regulate the flow of fuel therethrough;

said signal applying means including two alternative paths for saidsignal, the first of said paths being operative during said ignitionperiod; means responsive to the level of said signal for effectingtransfer of said signal to the second path when said signal reaches apredetermined level; and

means associated with said second path for maintaining a minimum signallevel corresponding to the minimum fuel flow required to maintaincombustion during said operating period.

2. The system recited in claim 1 wherein the second alternative pathcarries the control signal during the operating period.

3. The system recited in claim 1 wherein the alternative paths have acommon portion and the means responsive to the signal level is connectedto said common portion.

4. The system recited in claim 1 wherein the means to effect transfer ofthe signal to the second path comprises a signal diverting device; andthe signal level responsive means activates the diverting device todivert the control signal to the second path, when the level of thesignal corresponds to the operating period of the turbine.

5. The system recited in claim 1 wherein a plurality of turbineparameters are detected and converted into signals, and furthermore,means are provided to process the information in the signals andgenerate an output signal which is a continuous function of the fueldemanded.

6. The system recited in claim 5 wherein the signals fed into the signalprocessing means are electrical and the fuel demand signal exitingtherefrom is electrical,

means for converting said demand signal from an electrical demand signalto a pneumatic demand signal, said pneumatic signal in communicationwith first and second alternative signal paths.

7. The system recited in claim 1 wherein the minimum signal maintainingmeans comprises:

a signal regulating device providing a minimum low level signal tomaintain combustion;

a low limit signal path;

a first one way signal blocking device secured to said low limit path;

a second one way signal blocking device secured to the second alternatepath;

said first blocking device preventing the low level signal in said lowlimit path from entering the second path, when the magnitude of thesignal in the second path is greater than the magnitude of said lowlimit signal;

said second blocking device preventing the signal in the secondalternate path from continuing therethrough, when the magnitude of thelow limit signal is greater than the signal in the second path.

8. The system recited in claim 1 wherein the fuel control signal is anelectrical signal,

means for converting said electrical signal to a pneumatic signal,

said signal level responding means electrically activating a signaldiverting device when the pressure level of the pneumatic signal reachesa level corresponding to the operating period, to switch the pneumaticflow in the signal paths therein, from the first path to the secondpath.

9. In a gas turbine comprising at least one combustion chamber and arotor structure, said rotor structure being rotatable at a low speedduring a cranking period, at an intermediate speed during an ignitionperiod, and at a high speed during an operating period, a system forcontrolling fuel supplied to said combustion chamber, said controlsystem comprising:

valve means for regulating the flow of fuel to said combustion chamberduring said ignition and operating periods;

means to detect the value of at least one turbine parameter and convertit into a fuel control signal which is a continuous function of the fueldemand of the turbine;

first and second alternate signal paths, having a common portion, saidpaths being adapted to carry said control signal to said valve means;

means for diverting said signal from said first path to said secondpath;

means for sensing the magnitude of said signal in said common portion,said sensing means activating said diverting mans;

means for maintaining a minimum control signal to the valve to insurethat the valve structure remains open to supply fuel to the combustionchamber at least at the minimum quantity needed to sustain combustionduring the operating period;

said minimum control signal maintaining means being associated with thesecond signal path.

10. The system recited in claim 9 wherein the sensing means activatesthe diverting means, when the magnitude of the demand signal reaches alevel corresponding to the operating period, to switch the signal fromthe first path to the second path, so that the first path carries thecontrol signal during the ignition period and the second path carriesthe control signal during the operating period.

111. The system recited in claim 9 wherein the minimum control signalmeans provides a predetermined minimum signal into the second path whenthe control signal falls below the value of the minimum signal.

12. The system recited in claim 9 wherein the minimum control signalmeans comprises:

a signal regulating device providing a minimum low level signal toprevent turbine flameout;

a low limit path connecting said device to the second alternate path;

a first one way signal blocking device secured to said low limit path;

13. The system recited in claim 12 wherein the minimum control signalmeans comprises a pneumatic system, wherein the signal regulating deviceprovides a predetermined pneumatic signal, the low limit path carriesthe pneumatic signal to the second alternate path,'

the second alternate path carries a pneumatic signal, and said first andsecond blocking devices are one-way check valves which block therespective signals when the pressure from the signal in the respectivepath is less than the pressure in the corresponding path.

1. In a gas turbine comprising at least one combustion chamber and arotor structure, said rotor structure being rotatable at a low speedduring a cranking period, at an intermediate speed during an ignitionperiod, and at a high speed during an operating period, and a system forcontrolling fuel supplied to said combustion chamber, said controlsystem comprising: valve means for regulating the flow of fuel to saidcombustion chamber during said ignition and operating periods; meansresponsive to at least one parameter of turbine operation forcontinuously determining the fuel flow required by said combustionchamber and for providing a signal representative of said fuel flow;means for applying said signal to said valve means to effectuateoperation of said valve means to regulate the flow of fuel therethrough;said signal applying means including two alternative paths for saidsignal, the first of said paths being operative during said ignitionperiod; means responsive to the level of said signal for effectingtransfer of said signal to the second path when said signal reaches apredetermined level; and means associated with said second path formaintaining a minimum signal level corresponding to the minimum fuelflow required to maintain combustion during said operating period. 2.The system recited in claim 1 wherein the second alternative pathcarries the control signal during the operating period.
 3. The systemrecited in claim 1 wherein the alternative paths have a common portionand the means responsive to the signal level is connected to said commonportion.
 4. The system recited in claim 1 wherein the means to effecttransfer of the signal to the second path comprises a signal divertingdevice; and the signal level responsive means activates the divertingdevice to divert the control signal to the second path, when the levelof the signal corresponds to the operating period of the turbine.
 5. Thesystem recited in claim 1 wherein a plurality of turbine parameters aredetected and converted into signals, and furthermore, means are providedto process the information in the signals and generate an output signalwhich is a continuous function of the fuel demanded.
 6. The systemrecited in claim 5 wherein the signals fed into the signal processingmeans are electrical and the fuel demand signal exiting therefroM iselectrical, means for converting said demand signal from an electricaldemand signal to a pneumatic demand signal, said pneumatic signal incommunication with first and second alternative signal paths.
 7. Thesystem recited in claim 1 wherein the minimum signal maintaining meanscomprises: a signal regulating device providing a minimum low levelsignal to maintain combustion; a low limit signal path; a first one waysignal blocking device secured to said low limit path; a second one waysignal blocking device secured to the second alternate path; said firstblocking device preventing the low level signal in said low limit pathfrom entering the second path, when the magnitude of the signal in thesecond path is greater than the magnitude of said low limit signal; saidsecond blocking device preventing the signal in the second alternatepath from continuing therethrough, when the magnitude of the low limitsignal is greater than the signal in the second path.
 8. The systemrecited in claim 1 wherein the fuel control signal is an electricalsignal, means for converting said electrical signal to a pneumaticsignal, said signal level responding means electrically activating asignal diverting device when the pressure level of the pneumatic signalreaches a level corresponding to the operating period, to switch thepneumatic flow in the signal paths therein, from the first path to thesecond path.
 9. In a gas turbine comprising at least one combustionchamber and a rotor structure, said rotor structure being rotatable at alow speed during a cranking period, at an intermediate speed during anignition period, and at a high speed during an operating period, asystem for controlling fuel supplied to said combustion chamber, saidcontrol system comprising: valve means for regulating the flow of fuelto said combustion chamber during said ignition and operating periods;means to detect the value of at least one turbine parameter and convertit into a fuel control signal which is a continuous function of the fueldemand of the turbine; first and second alternate signal paths, having acommon portion, said paths being adapted to carry said control signal tosaid valve means; means for diverting said signal from said first pathto said second path; means for sensing the magnitude of said signal insaid common portion, said sensing means activating said diverting mans;means for maintaining a minimum control signal to the valve to insurethat the valve structure remains open to supply fuel to the combustionchamber at least at the minimum quantity needed to sustain combustionduring the operating period; said minimum control signal maintainingmeans being associated with the second signal path.
 10. The systemrecited in claim 9 wherein the sensing means activates the divertingmeans, when the magnitude of the demand signal reaches a levelcorresponding to the operating period, to switch the signal from thefirst path to the second path, so that the first path carries thecontrol signal during the ignition period and the second path carriesthe control signal during the operating period.
 11. The system recitedin claim 9 wherein the minimum control signal means provides apredetermined minimum signal into the second path when the controlsignal falls below the value of the minimum signal.
 12. The systemrecited in claim 9 wherein the minimum control signal means comprises: asignal regulating device providing a minimum low level signal to preventturbine flameout; a low limit path connecting said device to the secondalternate path; a first one way signal blocking device secured to saidlow limit path; a second one way signal blocking device secured to thesecond alternate path; said first blocking device preventing the lowlevel signal in said low limit path from entering the second path, whenthe magnitude of the signal in the second path is greater than themaGnitude of said low limit signal, said second blocking devicepreventing the signal in the second alternate path from continuingtherethrough, when the magnitude of the low limit signal is greater thanthe signal in the second path.
 13. The system recited in claim 12wherein the minimum control signal means comprises a pneumatic system,wherein the signal regulating device provides a predetermined pneumaticsignal, the low limit path carries the pneumatic signal to the secondalternate path, the second alternate path carries a pneumatic signal,and said first and second blocking devices are one-way check valveswhich block the respective signals when the pressure from the signal inthe respective path is less than the pressure in the corresponding path.